In our co-pending patent application WO 9706619, there is described a procedure for suppressing near-end crosstalk (NEXT) when using duplex communication, in twisted pair wire networks, in which different subcarriers are used in the two transmission directions, and in which the subcarriers are orthogonal. Preferably a number of sub-carriers are transmitted in each transmission direction. Alternate sub-carriers are used in each direction. The number of sub-carriers used in one transmission direction may be greater than the number used in the other transmission direction.
In our co-pending patent application, referenced above, the use of Zipper VDSL with time synchronization is described. The present invention complements the earlier invention, in that it enables VDSL Zipper duplex transmission, as described in our co-pending patent application, without time synchronization between different modem pairs over the same twisted copper pair cable, i.e. in the same binder group.
Zipper is a time-synchronized frequency division duplex implementation of discrete multi tone (DMT) modulation. Two communicating Zipper modems transmit DMT symbols simultaneously with a common clock. The Zipper scheme implies that every carrier, in the total set of carriers in the DMT signal, is exclusively chosen to be used for either for the upstream, or the down-stream, direction. When all transmitters are time synchronized, the near end cross-talk (NEXT) and near end echoes, injected into the received signal, are orthogonal to the desired signal. To ensure the orthogonality between the signal and all the noise sources originating from DMT signals in the opposite direction, the guard time, with cyclic extension of the symbols between consecutive symbols, must be dimensioned for the maximum propagation delay of the channel. Further, the size of the guard time is minimised by applying timing advance.
Several duplex VDSL systems may share the same twisted pair copper cable. Such systems are referred to as belonging to the same binder group. VDSL systems, using Zipper, which belong to the same binder group, are affected by line attenuation, near end echo and crosstalk. In known Zipper transmission systems, when timing advance is used, all transceivers, in a binding group, start the transmission of each frame at the same time.
There are three types of signal that affect the length of the cyclic extension in each frame:                the received signal;        the echo signal caused by imperfect balance of the hybrid and impedance discontinuities in the line; and        the NEXT signal.        
Orthogonality, between the received signal, the echo signal and the NEXT signal, is preserved if each sampled DMT symbol is disturbed by a single frame from each one of the near-end transmitters. Because of this, in order to preserve orthogonality, the cyclic extension has to be dimensioned to cover all impulse responses from the line, the echoes and the NEXT.